clean up imports in IdentifierParser
rename 'literal' -> 'extension literal' where appropriate

so it's clearer when we're talking about all literals (which
includes things like strings and numbers) and when we mean
extension literals specifically.  extension literals being
things like '{{array: ...}}' that appear in exported worlds
rename TokenType.Constant to TokenType.Literal

it's a peculiarity of NetLogo that named constants like `e` and `pi`
are actually literals rather than 0-argument reporters.  (in the old
days we didn't have ConstantFolder, so there would have been a
performance price to pay for having them be reporters.)  but I think
it's clearer, less surprising and more standard, namingwise, to
subsume named constants as a special case of literals, rather than
vice versa.